atob.lua

-- Lua uses at least 8 bytes to represent a number
-- Base64 minimum processable unit is represented by four 6-bit chunks which is 24-bits => 3 bytes
-- Hence we can safely use bit operations on number type for multiplexing and extraction

-- Define function to extract 8-bit ASCII char from 24-bit base64 unit
local function extractChar(unit, idx)
	-- Skip mask for first ASCII char since shifting will destroy other two parts
	if idx == 2 then
	  local charCode = (unit) >> (8 * idx)

	  return string.char(charCode)
	end

	-- Extract specific 8-bit
	local mask = (2^8 - 1) << (8 * idx)
  
	-- Move 8-bit to the beginning so it becomes a char code
	local charCode = (unit & mask) >> (8 * idx)

	-- Convert to string
	return string.char(charCode)
  end
  
  -- Define mapping of base64 character to number
  local mapping = {
	  A=0, B=1, C=2, D=3, E=4, F=5, G=6, H=7,
	  I=8, J=9, K=10, L=11, M=12, N=13, O=14, P=15,
	  Q=16, R=17, S=18, T=19, U=20, V=21, W=22, X=23,
	  Y=24, Z=25, a=26, b=27, c=28, d=29, e=30, f=31,
	  g=32, h=33, i=34, j=35, k=36, l=37, m=38, n=39,
	  o=40, p=41, q=42, r=43, s=44, t=45, u=46, v=47,
	  w=48, x=49, y=50, z=51, ["0"]=52, ["1"]=53, ["2"]=54, ["3"]=55,
	  ["4"]=56, ["5"]=57, ["6"]=58, ["7"]=59, ["8"]=60, ["9"]=61, ["+"]=62, ["/"]=63
  }
  local padding = "="
  local missing = ""
  
  local function atob(encodedString)
    -- Get four 6-bit chunks which forms one unit
  	-- There must be at least two 6-bit chunks to cover at least one 8-bit chunk
	local decodedString = encodedString:gsub("(.)(.)(.?)(.?)", function(c1, c2, c3, c4)
	  -- Convert each 6-bit chunk to number
	  local b1 = mapping[c1] --24,23,22,21,20,19
	  local b2 = mapping[c2] --18,17,16,15,14,13
	  local b3 = mapping[c3] --12,11,10,09,08,07
	  local b4 = mapping[c4] --06,05,04,03,02,01

	  -- Set optional chunks to 0 to be able to multiplex
	  if b3 == nil then b3 = 0 end
	  if b4 == nil then b4 = 0 end
  
	  -- Multiplex onto 24-bit number
	  local muxedUnit = (b1 << (6 * 3)) + (b2 << (6 * 2)) + (b3 << (6 * 1)) + (b4 << (6 * 0))
	  
	  -- Extract 8-bit chunks
	  -- Use empty string if unit is not full or it is padded
	  -- and handle A base64 characters at the end of encodedString
	  local r1 = extractChar(muxedUnit, 2)
	  -- if c3 is padding or missing then we can skip r2
	  local r2 = (c3 == padding or c3 == missing) and "" or extractChar(muxedUnit, 1)
	  -- if c4 is padding or missing then we can skip r3
	  local r3 = (c4 == padding or c4 == missing) and "" or extractChar(muxedUnit, 0)

	  -- Concatanate
	  return r1..r2..r3
  end)

  return decodedString
end

return atob

-- Test plan
assert(atob("bGlnaHQgd28=") == "light wo", "full units, single padding")
assert(atob("bGlnaHQgd28A") == "light wo\x00", "full units, explicit A")
assert(atob("bGlnaHQgd28") == "light wo", "2/3 of unit")
assert(atob("bGlnaHQgdw==") == "light w", "full unit, double padding")
assert(atob("bGlnaHQgdw") == "light w", "1/3 of unit")
assert(atob("bGlnaHQgd28=") == "light wo", "full units, single padding")
assert(atob("bGlnaHQgd29y") == "light wor", "full units, no padding")